He has called for more flexibility, allowing Scotland to have a different rules from the rest of the UK if Scots vote No in September's independence referendum.

McConnell wants radical changes to the UK system that would allow Scotland to set its own immigration rules and welfare policy with agreement from the Westminster Government.

In a new book of essays on tough issues, McConnell calls for a debate on how the UK is governed if there is a referendum No vote.

“It is possible for different parts of the UK to experiment with different kinds of policy, like the smoking ban, to prove they can work and then be transferred to other parts of the country,” he said.

“On welfare policy, on immigration rules and economic development, the UK Government should be able to embrace the potential of that.”

His comments will delight the SNP who argue that an independent Scotland can have a different immigration policy from the UK without border controls being in place.

Yesterday McConnell said: “When I was First Minister we had a policy to encourage talented people to move to Scotland. It didn’t mean that people fiddled the immigration system, we worked with David Blunkett as Home Secretary to make sure it worked.

“There is no reason why we can’t have that flexibility within the UK system.”

He also wants to see the Scottish Secretary post abolished and the Barnett public spending formula abandoned in a series of reforms that go much further than his own party’s devolution plans.

The intervention comes as Labour’s Johann Lamont, Tory leader Ruth Davidson and Lib Dem Willie Rennie today pledge to work together to mark their joint support for more devolution in the UK.

Lamont said: “We are now clear: a No vote does not mean no change. A No vote opens the door to more powers for Scotland. And if people do vote against independence, we can get on with the job of reforming the Scottish Parliament for the better.”